Moreover, during the night of Aug 13th, it was possible to perform the
first spectropolarimetric measurement, yielding a mean polarization level
P = (2.9+-0.1)%, with only slight wavelength dependance (Barth et al., GCN 1477).
The night after, another polarimetric observation (V-band) was performed
with the ESO VLT-UT3 (Melipal), resulting in P = (1.17 +/- 0.16)%
((0.80 +/- 0.16)% after correcting for Galactic-induced polarization; Covino
et al., GCN 1498).
This is the first unambiguous highly-significant detection of polarization
variability in a GRB afterglow.

Observations.

In the night between Aug 13 and Aug 14 UT, we observed the optical afterglow
of GRB 020813 with the ESO 8.2m VLT UT3 (Melipal), in polarimetric
mode. The subsequent nights, we activated the Italian 3.6m Telescopio Nazionale
Galileo (TNG), beginning in the night
of Aug 14th, to perform broad band photometry (ranging from U to K)
and to sample the light curve. We also used the Campo Imperatore
AZT24 1.1m telescope (near L'Aquila, Italy) to get additional near-infrared
measurements. We also reanalyzed the publicly available
data from Mike Gladders and Pat Hall (GCN 1519), taken
with the Baade 6.5m (Magellan 1)
Telescope during the night of Aug 13th and 14th. The observation log is reported
here.

The picture shows the field (1'x1') of GRB 020813 afterglow (indicated
by the arrow), as imaged by TNG on Aug 15.0 UT. This is a composite image
from separate B, V and R exposures.

Data Analysis.

Collecting our multiband photometry (left panel in the figure), we could
construct the light curve of the event for several bands, which shows a sharp
achromatic break about 14 h after the GRB. Parametrizing the light curve
as a broken power law (F(t) = kt-d),
we got asymptotical slopes d1 = 0.78 and d2
= 1.44. The break is very sharp, as it can be seen by analyzing early time
data (t < 5 h; right panel), which do not show signs of curvature.

To further check if the break is indeed achromatic, we performed spectral
analysis at different times, both before and after the break. We found that
during all our observations the spectrum was well described by a powerlaw
with spectral index a = 1.04 (once correcting for Galactic extinction). The
figure on the left shows all our photometric spectra. Collecting measurements
in other bands (GCNs 1487, 1490, 1497, 1504), we could also
construct the spectral energy distribution of the afterglow from radio to
X-rays (figure on the right). These data suggest that the optical light curve
may suffer some extinction within the host galaxy.

Photometric data.

Here are reported all numbers from our measurements. Original data are available
upon request (contact: Stefano
Covino).

Polarization analysis.

With the VLT UT-3 (Melipal), we imaged the optical afterglow of GRB 020813,
in the V-band. We found a positive level of polarization, with P
= (0.80 +/- 0.16)% (after correcting for Galactic-induced polarization). This
is one of the smallest values ever observed for a GRB afterglow, yet determined
with good accuracy. Coupled with the measurement of Barth et al. (GCN 1477), this is
also the first unambiguos higly-significant detection of polarization variability
in a GRB afterglow. The figure shows the position of the afterglow (red point)
and of some field stars (blue points) in the Stokes Q-U plane.
The transient clears stands outside the region occupied by (slightly polarized)
stars.